One thing that I was very surprised to see in The faq was that ynarri cannot "gain battle focus by any means"
I understand this ruling for full time rule bonuses such as the wraith host that straight up gets battle focus, or that special harlequin troupe that has battle focus.
What really disappointed me I guess was that, without including such a rule in the errata, GW nullified the effects of the Iyanden runes of battle primaris psychic power, a blessing that temporarily grants battle focus to a very specific variety of units.
I am left wondering if this was intentional, or just a by product of preventing units from gaining battle focus from things like the wraith host formation.
Not to beat a dead horse, but I guess I just think it's odd that GW would make a certain psychic power straight up bot work on the units it was intended to work on.
They also don't sell that book anymore, so I don't think they really put it into consideration either. Is the supplement event legal to use anymore? I thought it wasn't.
If there is a power they have, it'll effect battle brothers just fine.
The rules for Ynnari says
However, units with the Ynnari Faction lose the Ancient Doom, Battle Focus and Power From Pain special rules, if they have them.
It's unclear if the model can 'regain' battlefocus during the game or not. I'd err on the side of they would lose the ability whenever they gain it.